The present invention generally relates to memory modules for installation in computer memory subsystems. More particularly, this invention relates to a memory module equipped with termination resistors located directly on or immediately adjacent the I/O pins of memory packages of the module to reduce noise and signal reflections.
The computer industry has moved to higher speed grades not only in the field of processor technology but also relating to all peripheral devices including the system memory. The latter has become the main bottleneck in the overall system performance in that, with increasing clock rates, the central processors are starved for data and more and more cycles are wasted idly because of the lack of data and instructions to be processed.
Memory clock and data frequency are limited primarily by two different factors, the first being the core and I/O design of the actual memory IC and the second being the interfacing with the rest of the system logic. In the case of the first, consumer-class graphics boards have shown that speed grades of 700-800 MHz data rate are within realistic expectations of high-grade components. However, one must bear in mind that this is achieved using a point to point connectivity without socketed interfaces.
One particular obstacle in the case of system memory is the interfacing of the memory devices themselves with the rest of the system. Such interfacing typically involves soldering memory components (e.g., packages) onto a substrate, such as a printed circuit board (PCB), forming what is commonly termed a memory module, and routing the signals in and out of the memory module through edge connectors on the PCB to a system board (mainboard or motherboard) using, for example, a dual in-line memory module (DIMM) socket (slot). This kind of interfacing necessarily involves the transition of all signals through a variety of interfaces that include both soldered and pressure contacts. Each transition from one medium to the next generates signal reflections that fill the bus with noise. If the noise levels reach a critical threshold, they will interfere with the signal integrity of the actual data, addresses, and commands and induce errors on several levels. Such degradation of signal quality can be of particularly concern with higher signal frequencies or with digital signals that exhibit shorter rise times. Where lower power supply voltages are employed, electrical resonances and reflections can also be of concern because lower noise is required to avoid detrimentally affecting signal integrity and electrical system reliability.
Suffice it to say that the amount and level of signal reflections are inversely correlated to the quality of the interface. Consequently, a major hurdle toward reaching clean signals is the socket interface formed by the edge connections of the memory module and the DIMM slot of the system board. The most common approach to reducing noise and increasing signal integrity as well as providing termination of the actual signal has been the use of termination resistors in signal paths. In the first generation DDR (or DDR-SDRAM—double data rate synchronous DRAM) architecture, termination is provided on the mainboard by means of a parallel resistor to the termination voltage (VTTI). One of the improvements of second generation DDR (DDR2) is the presence of the so-called on-die-termination (ODT). Briefly, ODT moves the point at which termination occurs as close to its source as possible—that is, the point of signal origin, which is the die itself. Because of this measure, the input/output (I/O) frequency can be increased since the improved termination scheme ameliorates shortcomings of the system PCB design and bus interface.
The most effective method for reducing noise and signal reflections is their elimination as close to their source as possible. As such, on-board (mainboard) termination is inferior to on-die termination, especially since the termination of all memory slots is shared and, by extension, the noise is shared between all modules in the system before it reaches the termination resistors. However, current die and package designs do not leave enough room to implement efficient termination in the package.